The present invention relates generally to an illumination apparatus, and more particularly to an illumination apparatus used to expose an object, such as a single crystal substrate for a semiconductor wafer, a glass plate for a liquid crystal display (“LCD”), an exposure apparatus using the illumination apparatus and a device manufacturing method using the exposure apparatus.
The recent demands for smaller and lower profile electronic apparatuses increasingly require finer processing to the semiconductor devices mounted onto these electronic devices. The photolithography process is generally used to produce highly integrated devices, such as a semiconductor device, a LCD device and a thin film magnetic head. A projection exposure apparatus is a vital apparatus to this process and exposes a pattern of a photo-mask (or reticle) onto a substrate, such as a photoresist applied silicon wafer and glass plate.
The following equation gives the resolution R of the projection exposure apparatus, where λ is a wavelength of the exposure light, NA is a numerical aperture of the projection optical system, and k1 is a process constant determined by a development process:
                    R        =                              k            1                    ×                      λ            NA                                              (        1        )            
Therefore, the shorter the wavelength and the higher the NA, the better the resolution. However, a shorter wavelength would disadvantageously reduce the transmittance through a glass material, the depth of focus decreases in inverse proportion to the NA, and a larger NA makes difficult a design and manufacture of a lens.
Accordingly, the resolution enhancement technology (“RET”) has been recently proposed which reduces a value of the process constant k1 for the fine processing. One RET is a modified illumination, which is also referred to as an oblique incidence illumination or an off-axis illumination. The modified illumination arranges an aperture stop with a light-shielding plate on an optical axis in an optical system near an exit surface of a light integrator that forms a uniform surface light source, and introduces the exposure light obliquely to a mask. The modified illumination can form an annular illumination, a quadrupole illumination, etc. by controlling a shape of the aperture stop.
On the other hand, prior art propose the modified illumination having only linearly polarized light in a desired direction for the enhanced image contrast. See, for example, Japanese Patent Applications, Publication Nos. (“JPs”) 7-183201 and 6-053120. JP 7-183201 uses a filter to remove the light if the light does not have a desired polarization direction. JP 6-053120 discloses a method for creating the linearly polarized light in a desired direction by using a λ/2 phase plate and the light that has been previously linearly polarized by a linear polarizer.